scylla_raid_setup: use mdmonitor only when RAID level > 0
scylla_raid_setup: use mdmonitor only when RAID level > 0

We found that monitor mode of mdadm does not work on RAID0, and it is
not a bug, expected behavior according to RHEL developer.
Therefore, we should stop enabling mdmonitor when RAID0 is specified.

Fixes #9540

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla